fix: resolve all clippy warnings and enforce cargo fmt

- display_helpers: convert module doc-comments to regular comments,
  merge identical text/markdown + text/ branches
- search_service: replace needless range loops with slice-based pagination
- folder_repository, folder_db_repository: collapse nested if statements
- favorites_pg_repository: remove unnecessary borrow on generic arg
- file_blob_read_repository: collapse 6 nested if-let blocks
- file_blob_write_repository: collapse nested if for dedup ref decrement
- chunked_upload_service: use div_ceil(), collapse 2 nested if blocks
- folder_handler: collapse nested if-let for owner check
- webdav_handler: replace 7x io::Error::new(ErrorKind::Other, ..) with
  io::Error::other(..)
- cargo fmt applied to all files

Passes: cargo clippy --all-targets --all-features -- -D warnings

src/application/ports/file_ports.rs

@@ -165,10 +165,7 @@ pub trait FileRetrievalUseCase: Send + Sync + 'static {
     /// Lists every file in the subtree rooted at `folder_id`.
     ///
     /// Default: falls back to `list_files(Some(folder_id))` (one level).
-    async fn list_files_in_subtree(
-        &self,
-        folder_id: &str,
-    ) -> Result<Vec<FileDto>, DomainError> {
+    async fn list_files_in_subtree(&self, folder_id: &str) -> Result<Vec<FileDto>, DomainError> {
         self.list_files(Some(folder_id)).await
     }
 
@@ -183,7 +180,11 @@ pub trait FileRetrievalUseCase: Send + Sync + 'static {
         limit: i64,
     ) -> Result<Vec<FileDto>, DomainError> {
         let all = self.list_files(folder_id).await?;
-        Ok(all.into_iter().skip(offset as usize).take(limit as usize).collect())
+        Ok(all
+            .into_iter()
+            .skip(offset as usize)
+            .take(limit as usize)
+            .collect())
     }
 }